The present invention relates to an acceleration sensor and a manufacturing method for the same, and more specifically, a mass production type acceleration sensor to be used for small-sized household electronic devices and a manufacturing method for the same.
Household electronic devices with a microprocessor installed inside, such as cell phones, digital cameras, electronic games, and PDAs have strongly spread, and recently, the demand for acceleration sensors to be installed in these electronic devices or input devices of electronic devices has also increased. In an electronic device having an acceleration sensor installed inside, acceleration components with an impact or a vibration applied to the device main body can be taken in the microprocessor as digital data, so that proper processing while grasping the physical environments around the electronic device becomes possible. For example, in the case of a digital camera, correction for camera shake can be carried out by detecting the acceleration that has acted at the moment when the shutter button is pressed. Furthermore, in an input device of an electronic game, it also becomes possible to input a user's operation instruction in the form of an acceleration. For example, when an acceleration sensor is used for detection of an gravity acceleration, the acceleration sensor can be used as a clinometer, and it becomes possible to recognize the azimuth of the entire electronic device (degree of inclination from the horizontal status), so that predetermined operation and input become possible based on a user's operation for inclining the entire electronic device.
It is desirable that an acceleration sensor to be installed inside such a small-sized household electronic device is small-sized and suitable for mass production. For example, Japanese Patent Publications No. 01-263576 and No. 03-202778 disclose structures of small-sized acceleration sensors suitable for mass production, and Japanese Patent Publication No. 04-249726 discloses a manufacturing method for mass production of such an acceleration sensor by using a silicon substrate. As such a general acceleration sensor, a type has been generally used in which a weight is joined to a flexible substrate, and the substrate is bent in response to an acceleration applied to the weight, and the bend of this substrate is electrically detected. For detecting the bend of the substrate, various detectors such as a piezo resistor element, a capacitor element, or a piezoelectric element have been used.
As mentioned above, as a small sized acceleration sensor suitable for mass production, a sensor using a semiconductor substrate such as a silicon substrate has been proposed. In order to increase the detection sensitivity in such an acceleration sensor, an increase in weight mass or an increase in flexibility of the weight supporting portion is required. However, in all cases where an excessive acceleration is applied, there is a possibility that the weight supporting portion is broken. Particularly, if the sensor casing drops by mistake, a great impact is applied to the weight, and there is a possibility that the weight supporting portion formed of a semiconductor is damaged. Therefore, it is required to provide a physical control structure for controlling displacements of the weight within a predetermined range under normal conditions. Also in the case of the acceleration sensors disclosed in the abovementioned publications, a physical control structure such as a control substrate or a pedestal is provided for controlling vertical and horizontal displacements of the weight. When an excessive acceleration acts, a part of the weight comes into contact with the control substrate or the pedestal, and this suppresses displacements of the weight within a predetermined range of freedom. Therefore, the weight supporting portion can be prevented from being applied with an excessive stress and being broken.
However, it is required that such a control structure has a predetermined shape and is disposed at a predetermined position in accordance with the shape and disposition of the weight. Therefore, in order to manufacture an acceleration sensor with a control structure, an etching step or machining step is additionally required, and this makes the manufacturing process complicated. Particularly, in order to secure uniform performance among lots to be manufactured as mass-produced items, it is required that the distance between the weight and the control structure is accurately set. Therefore, in the conventional acceleration sensors disclosed in the abovementioned publications, a great technical burden is imposed on the step for forming the control structure, and this also poses a problem in terms of cost reduction.